The present invention relates to a system for demodulating a frequency- or phase-modulation signal, and more particularly a system for digitally demodulating the received modulation signal by quantizing phase information of a fixed period detected by using two local frequency signals of quadrature-phase, wherein the two local signals have the same frequency as the carrier wave of the modulation signal.
Generally, the FM(Frequency Modulation) receiver demodulates the modulated signal by using a superheterodyne system as shown in FIG. 1. In the superheterodyne system, first mixer 4 mixes the received RF(Radio Frequency) signal with a first local oscillation signal LS1 generated by a first local oscillator 3 so as to form a first intermediate frequency (IF) signal, and first IF filter eliminates unwanted signals such as harmonics, etc. produced when the received RF signal and the first local oscillation signal are mixed in the first mixer 4. Also, the first IF filter generates the first IF signal IF1. Additionally, a second mixer 7 mixes the IF signal IF1 with a second local oscillation signal LS2 generated by a second local oscillator 6 to form a second IF signal IF2, and a second IF filter 8 eliminates the adjacent frequency in order to sensitively distinguish the adjacent channel, outputting the second IF signal IF2. Here, the first IF signal IF1 is usually a high frequency of several tens to several hundred MHz and the second IF signal IF2 is a relatively low frequency. An FM demodulator 9 demodulates the second IF signal IF2 to reproduce the original message signal. Because the conventional superheterodyne system as described above demodulates the received RF signal by using two local oscillators, automatic frequency control (AFC) is hardly realized to control the frequency deviation between the input signal and the local oscillation signal, the first IF filter requires a large size and high cost in order to filter the frequency of several tens of MHz. Furthermore, the conventional method makes it difficult to obtain a constant group delay characteristic according to frequencies used in data communication, and has a problem in integration and reduction of the system.
Another method or demodulating a frequency- or phase-modulated signal is as shown in FIG. 2, which illustrates an FM detecting system of analog method using quadrature phase, disclosed by I. A. W. Vance. (see, I. A. W. Vance, An Integrated Circuit VHF Radio receiver, proceedings of the IERE Conference on Land, Mobil Radio, Sep. 4-7, 1979). This system comprises an antenna 11 for receiving a modulated signal, a local oscillator 13 for generating a signal of the same frequency as the carrier wave of the received RF signal, a phase shifter 14 for shifting, by 90.degree., the phase of the local oscillation frequency of said local oscillator 13, an in-phase mixer 15 (hereinafter referred to as I-mixer) for mixing the received RF signal and the local oscillation frequency outputted through said phase shifter 14, a quadrature-phase mixer 16 (hereinafter referred to as Q-mixer) for mixing the received RF signal and the local oscillation frequency of the local oscillator 13, first and second low-pass-filters (hereinafter referred to as LPF) 17, 18 for filtering the outputs of the I-mixer 15 and Q-mixer 16 into a half bandwidth of the RF channel band, first and second variable gain amplifiers 19, 20 for variably amplifying the outputs of the first and second LPFs 17, 18, a level detector 21 for detecting the output level of the first and second variable gain amplifiers 19, 20, to deliver an automatic gain control (AGC) signal to the first and second variable gain amplifiers 19, 20, a first differentiator 22 for differentiating an I-channel gain control IF signal of the first variable gain amplifier 19, a second differentiator 23 for differentiating a Q-channel gain control IF signal of the second variable gain amplifier 20, an I-multiplier 24 for multiplying the I-channel gain control signal of the first variable gain amplifier 19 and the Q-channel differentiated signal of the second differentiator 23, a Q-multiplier 25 for multiplying the Q-channel gain control signal of the second variable gain amplifier 20 and the I-channel differentiated signal of the first differentiator 22, and a subtractor 26 for performing a subtracting operation from the outputs of said I-multiplier 24 and Q-multiplier 25, thereby outputting a demodulation signal.
In such a demodulation system of the quadrature-phase or -frequency modulation signal using analog method, the phase error originating from the two channels is outputted as its cosine function signal affecting only the amplitude thereof. But if non-equilibrium exists between the I-multiplier 24, Q-multiplier 25, and subtractor 26, the output signal is distorted. Furthermore, since the signals I(t) and Q(t) have a wide dynamic range from direct current (DC) to cutoff frequency of the LPF, it is difficult to realize the operation AGC through ideally detecting the level. Also, all the amplitude variations produced following the AGC operation turn into AM (Amplitude Modulation) noises. On the other hand, since all the steps are processed by the analog method, there appears a problem that the two channels must always be precisely matched with each other in its original integration, thereby decreasing its reliability.